This invention relates to a new and improved broadcast system, and more particularly, a frequency modulation broadcast system in which four discrete stereophonically related audio frequency inputs are transmitted and received.
It is well recognized that the realism and listening pleasure associated with broadcast or recorded music and other material can, in many instances, be increased substantially by providing a plurality of separate channels or audio inputs which are supplied to different speakers. Accordingly, two channel stereophonic systems have become commonplace, and most record discs and magnetic tape recordings are readily available in two channel stereophonic form. In addition, two channel stereophonic material is broadcast in accordance with standards that have been established by the Federal Communications Commission. A two channel stereophonic system of the type which has been adopted and standardized by the Federal Communications Commission is disclosed in my U.S. Pat. No. 3,122,610, issued on Feb. 25, 1964. It utilizes a first frequency band within which a main carrier wave is modulated with the sum of the left and the right channels. This main carrier wave is further frequency modulated with the sidebands of a suppressed subcarrier wave at 38 KHz that has been amplitude modulated with the difference between the left and right channels. A pilot signal is provided at 19 KHz within a gap between the two frequency bands to provide a basis for the local regeneration of the subcarrier in the receiver and to provide an indication of the presence of a stereophonic signal. This highly successful system is fully compatible with the prior monophonic, frequency-modulation broadcast systems.
It is now recognized that there are many advantages to a four channel stereophonic system in that it provides increased realism and listening pleasure as compared to a two channel system. This is particularly true, for instance, when the sound of a large concert hall is to be recreated. In that environment, the sound comes to the listener from many directions. A large part of this sound is reflected, thus introducing time delays which form a significant part of the listening experience. Four channel stereophonic music has been recorded on magnetic tapes and reproduced through speaker systems with good results. In addition, there has been some limited FM broadcasting of four channel stereophonic music utilizing two separate stations which are assigned different carrier frequencies.
It is important that a four channel stereophonic system be fully compatible with the large quantity of existing monophonic and two channel stereophonic equipment. If complete monophonic and two channel stereophonic information is to be provided for this equipment, the presently established sum and difference signals and the presently established 19 KHz pilot signal must be incorporated in the four channel system. Thus, the information needed to further break down the two existing stereophonic channels into four channels must be superimposed upon the established two channel stereophonic composite signal. It has not heretofore been known how to accomplish this objective without producing unacceptable out-of-band radiation.
There are a number of presently known stereophonic receivers which produce what may be termed a pseudo or hybrid four channel output. This is accomplished by matrixing the two conventional stereophonic inputs in the receiver, sometimes with the addition of time delays and loudness enhancement, to produce four inputs each of which may be different from the other three. These are not, however, four discrete inputs. They are four artificially created inputs, and the relationship between the inputs to the speakers is determined according to a formula which is preselected at the time the receiver is built. Some known systems utilize matrixing of four audio inputs at the transmitter, but only two channels are broadcast by the transmitter. However as in other hybrid systems, four channels of information are not broadcast by the transmitter, and the receiver is not equipped to detect this much information if it were present. Thus, the presently known four speaker receivers are inherently inferior because they are not part of an integrated system, including a transmitter and at least one receiver, designed to broadcast four discrete audio inputs.